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January 3rd 04, 10:39 PM

"Keven Matthews" a Ã©crit dans le message de news:
...
I recently moved my shack from an upstairs room to downstairs, much
closer
to the garden and antennas. All the antennas previously had a long run of
coax to the old shack. The obvious thing was to have a nice new short
run
of coax to my HF vertical which is now only 15' away. So I cut the coax
and
since then the antenna is no longer resonant on 40 Metres. Also this week
I
was putting up a new HF wire antenna, it was getting dark and raining by
the
time I was hoisting it up but so I could just have a listen that night a
grabbed an old (15 years) large coiled up of quantity RG213 coax
complete
with rotten oxydized pl259 plugs on each end which had just sat on the
garage wall for years. I just slung the coil down and plugged in at each
end. The plugs looked so rotten it was shameful but it pitch dark by
then!
However The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all
ready
for the following day. Guess what ? I put on the nice new cable and
plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting
on
my patio rather that a much shorter brand new piece. Please could some
one
explain to me if the coax length does matter, it has certainly never been
a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable
if
you phisically dont need it ?

Many Thanks & 73 for 2004

Keven G7UUD

Hello and happy new year.

It would always be necessary to tune the coaxial feeder
to the frequency of work.

For your problem it is necessary :

To measure the ros in one 1/2 length of cable feeder.
To tune the coaxial fededer with the frequency of work
on multiple of one 1/2 wavelength.
[Certain vertical antenna (as 5/8 lambda) have necessarily
to have 1 wavelength of cable feeder at least].
To control the ros and connect your installation.

Although it is it, value read in one 1/2 longuer of wave is
the same that that read to all the multiple of 1/2 lamda (3 or 10.5 lamda
for example).

Good luck

GG, amateur of radio for 19 years

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